Having trouble viewing this email? View it online here
Over the last 75 years we have helped our clients deliver cutting edge and innovative designs. We have put together our Engineers Guide To Spring Technology to share some of the common design problems that our designers have worked through. Please let us know if there are any other design problems or questions that you would like answering. We hope you enjoy the first edition!
Q: What purposes can stock springs be used for?
A: They are suitable for all non-critical engineering applications where there is no specific requirement for the length of life, or relaxation of the springs. If you have an application that demands either of these, please consult our Technical Department for advice. A well-designed bespoke spring can often be more cost-effective in the long run than using a stock item which is unsuitable for the conditions of use, and which fails as a result.
Q: Spring steel or Stainless steel; which should I choose?
A: Spring steel generally gives higher forces from a given space envelope than stainless steel. If your spring doesn't need to be corrosion resistant, spring steel products are usually significantly cheaper. Using pre-galvanised wire or Zinc plating the spring will give some corrosion protection. On the other hand, stainless steel springs, in addition to resisting rust, can also work at higher ambient temperatures (up to 300°C).
Q: What if I need my springs to work at temperatures higher than 300°C; is it possible?
A: Yes. It's possible to go up to 550°C by using an appropriate Nickel alloy material.
Q: I need to use spring steel for stress reasons in my tension spring, but I need some corrosion protection; what do you recommend?
A: A pre-galvanised wire as in this way there is a coating on the surfaces between the coil.
Q: I need a compression spring that will perform at least 10 million cycles; is that a realistic aim?
A: It is. Given the space to come up with a conservative design, we can arrive at a spring that will last almost indefinitely. We will use high grade materials with good surface finishes, and we will often choose to shot peen the springs to enhance their fatigue life.
Q: I want to compress my springs to solid under a specific force, is that OK?
A: Not really. As a spring approaches its solid length, some of the coils will touch neighbouring coils. The effect of this is to reduce the length of active material in the spring and hence stiffen the spring rate, leading to unpredictable force readings. The general consensus is to not specify forces in the last 20% of the spring's travel;-15% as an absolute minimum.
Q: What material should I use for battery contact springs?
A: You need a material that is a good conductor of electricity such as Beryllium Copper or brass. These are relatively expensive alloys however, so spring steel with a Nickel plated finish is frequently used in these instances.
Q: How do I specify the length of a tension spring with loops; do I measure over the coils or over the loops?
A: There are several ways to measure the length of a tension spring. Choose the one which best controls the length for the way in which you are using it. Over the coils is referred to as 'b/l' (body length), to the centre of the loops is 'c/c (centre to centre), and to the loop extremities is called 'o/a (overall). Perhaps the most popular way though, is to specify just inside the loops, which is known as 'p/p (pull to pull). Since this is normally where the spring is anchored, it's often the most appropriate.
Q: What is meant by the term 'initial tension'?
A: The term is used to refer to the amount of 'pre-tension' coiled into a tension spring;- it's the force which holds the coils together. This can be varied within certain limits. The maximum amount depends upon the stress due to the initial tension, and the spring index. It's actually very difficult to produce a close-coiled spring with no initial tension; attempting to do this results in some gaps between the coils.
Q: Does it matter whether torsion springs are coiled right hand or left hand?
A: In most cases, yes. You should normally arrange your torsion spring so that it 'winds up' when the torque is applied, i.e. that the number of coils is increasing as the force is applied. If the spring 'unwinds' when loaded, the maximum safe stress is very limited (about 30% of tensile strength).
Q: Is it possible to predict the fatigue life of a torsion spring?
A: Usually not, because most torsion springs rely on support from a mandrel in use. The friction between the inside diameter of the coils and the mandrel will usually be the eventual cause of failure. It is possible to arrange torsion springs so that the coils do not need support and in these cases very long fatigue lives can be achieved.
Q: What's the situation regarding fatigue life for tension springs
A: A conventional tension spring has a loop at each end and this loop is usually much more highly stressed than the coils in the body. As a result, most failures occur in the loops. If you need a tension spring with a reasonable fatigue life it's therefore necessary to reduce the stress in the loops or remove the loops altogether. We have a number of ways of achieving this so contact our Technical Department for advice.